A tube expander of this kind for expanding tubes used for a heat exchanger is known from, for example, Japanese Utility Model Publication No. 1-23650.
The tube expander for expanding tubes used for a heat exchanger disclosed by this publication is configured, as shown in FIG. 4, by a pair of support columns 19a that are formed on a base 19 with a predetermined distance, and the support columns 19a are connected with one another through a beam 20 at their tops, thereby creating an expander main frame 14a. 
The expander main frame 14a for heat exchanger tubes presses tube expander mandrels 5 into openings of heat exchanger tubes 16 of a heat exchanger 15 that is set on the base 19, thereby integrally forming an end plate 15a and a plurality of heat dissipation fins with the heat exchanger tubes 16. For doing this, the tube expander includes a reciprocal movement body 7 having the above noted expander mandrels 5 thereon which is inserted in guide bars (guide post) 21 formed in front of the support column 19a in a manner that is up/down movable (reciprocally movable) through two cylinders 6.
Further, on the reciprocal movement body 7, there is provided with a pinion gear 12 that is fitted with a rack 11 vertically formed and an encoder 13 for counting a rotation angle of the pinion gear. The pinion gear 12 and the encoder 13 measure a descending distance of the reciprocal movement body 7 that moves toward the heat exchanger 15.
Further, under the reciprocal movement body 7, a stripper plate 9 having strippers 9a is provided which contacts the end plate 15a of the heat exchanger 15 and presses down the end plate 15a for defining a projection length of the heat exchanger tubes 16 which are projected from the end plate 15a. 
On the stripper plate 9, a limit switch 10 is provided for detecting a start point of the downward movement of the stripper plate 9 that descends toward the heat exchanger 15 along with the reciprocal movement body 7.
Further, at both sides of the stripper plate 9, balance cylinders 8 are provided. The above noted encoder 13 measures the descending distance of the reciprocal movement body 7 in response to a detection signal from the limit switch 10 as a start signal. When the descending distance reaches a predefined value, the balance cylinders 8 operate to stop the downward movement of the stripper plate 9.
Therefore, in the tube expander for heat exchanger tubes configured in the manner described above, when the value that is measured through the encoder 13 in response to the detection signal from the limit switch 10 as the start signal reaches the predefined value, the balance cylinders 8 operate to stop the downward movement of the stripper plate 9, thereby properly controlling the length of the heat exchanger tubes 16 projected from the end plate 15a of the heat exchanger 15.
However, the tube expander for heat exchanger tubes configured in the manner described above has a problem in that a production cost will increase because various equipment such as the limit switch 10, the rack 11, the pinion gear fitted with the rack 11, and the encoder 13 have to be individually attached to the tube expander.
Accordingly, for suppressing the increase of the production cost, a tube expander for heat exchanger tubes as shown in FIG. 3 has been developed which is expected to replace the above described tube expander in the market.
In this tube expander for heat exchanger tubes, a cylinder 4 equipped with a pulse encoder 18 is utilized and a reference stand 1 is attached to an expansion and contraction rod 4a of the cylinder 4 through a fixing member 1a such as a male screw for stopping the downward movement of the stripper plate 9 where the reference stand 1 is coupled to the stripper plate 9 through a slide body 9b. 
Therefore, according to the latter tube expander for heat exchanger tubes, it is unnecessary to measure the actual downward distance of the reciprocal movement body 7 and to operate the balance cylinders 8 as required in the former tube expander for heat exchanger tubes, since the projection length of the expansion and contraction rod 4a can be set easily through the pulse encoder 18.
Accordingly, the production cost will be decreased because it can eliminate various equipment such as the limit switch 10, the rack 11, the pinion gear fitted with the rack 11, and the encoder 13.
However, the tube expanders for heat exchanger tubes in the conventional technology described above involve the following problems:
In either the former or latter structure described above, the stripper plate 9 is stopped at the predetermined position through only the pair of cylinders that support the both ends of the stripper plate 9.
Thus, when stopping the stripper plate 9 that moves downwardly along with the reciprocal movement body 7 for the tube expansion operation, only the expansion and contraction rod always receives the weight of the stripper plate 9 directly through the reference stand 1 attached to the expansion and contraction rod.
Accordingly, in the situation where only the expansion and contraction rod receives the weight of the stripper plate 9, when the expansion and contraction rod expands greater than a certain degree, the expansion and contraction rod will be instantaneously deformed. A degree of such deformation will increase in proportion to the expansion length of the expansion and contraction rod.
Therefore, in the case where the tube expansion operation is conducted for a heat exchanger 15 whose total length is especially large on the base 19 of the expander main frame 14a, the expansion and contraction rod, when in its maximum extended state, may be extended further than the cylinders. As a result, at the moment when the expansion and contraction rod at its maximum extended state receives the weight of the stripper plate 9, there arises a possibility that the expansion and contraction rod is greatly deformed which damages the sealing of the cylinders.
Further, in the case where reciprocal movement means is formed by a rod shaped male screw (not shown) established on the base 19 instead of the cylinders noted above for up-down movement of the reference stand 1, there arises a possibility that the rod shaped male screw is greatly deformed similar to the above example and damages threads of the screw.
To solve these problems, the applicant has invented the technology which is disclosed by Japanese Patent Application No. 2000-141590.
In this technology, a part of the reference stand is slidably attached to the above-mentioned support columns through a slide unit. This structure makes it possible that the deformation of the expansion and contraction rod or the rod shaped male screw is decreased when the reciprocal movement means of the reference stand, i.e., the expansion and contraction rod or the rod shaped male screw, receives the weight of the stripper plate instantaneously. Thus, it is expected that the damages to the sealing of the cylinders or the brakeage of the threads of the rod shaped male screw can be effectively avoided.
However, in this arrangement where the part of the reference stand is slidably attached to the above-mentioned support columns through the slide unit, a part of the slide unit will be worn down. This is especially true when the tube expander for heat exchanger tubes is established in a poor environment where dusts and dirts are in the air or when the tube expander for heat exchanger tubes is extremely frequently used, friction arises at the slide unit every time when the tube expansion operation is performed. As a result, it becomes impossible to smoothly move the reference stand and has to replace the reference stand. This not only increases the cost but also requires complicated works for removing the reference stand from the cylinder or the rod shaped male screw and attaching the new reference stand.
The present invention has been made in view of the above problems. It is an object of the present invention to provide a tube expander in which a part of the reference stand is slidably attached to the support column through a slide unit. In this tube expander, only a sliding part of the slide unit of the reference stand can be replaced easily at low cost with a new one without changing the reference stand even when the tube expander is established in the poor environment where dusts and dirts are in the air or when the tube expander for heat exchanger tubes is extremely frequently used.